Recently-used communications within vehicles have remarkable progress in networking.
Engine control, control of door mirrors and power windows, etc. are carried out by connecting between a plurality of control units in the network to enable the plurality of control units to exchange required information with one another.
Furthermore, construction of an in-vehicle network is in progress also for data in an information system which are information other than control information for vehicles.
For example, in a case of displaying a video image of a DVD player disposed for a front seat on an screen of a backseat, and in a case of outputting a sound from a speaker disposed at any one of places in the vehicle, the wiring number can be reduced by introducing networking rather than connecting between related devices individually.
Generally, the former network is referred to as a control system network, and the latter network is referred to as an information system network. Because requirements for networks, such as reliability and bandwidth, differ between the control system network and the information system network, they are constructed as physically-separate networks.
On the other hand, fusion of the control system network and the information system network contributes to the enhancement of the vehicle system greatly.
For example, thinking about a system which carries out control of a brake or a steering according to a video image captured by a vehicle-mounted camera, it is necessary to propagate data which are transmitted to an information system network to a control system network via a gateway device.
However, as mentioned above, because the control system network is a system which requires high reliability, simple filtering of the data which are transmitted to the information system network to flow the data into the control system network is inadequate. More specifically, it is necessary to take a measure of not obstructing communications in the control system network.
In the case of CAN (Controller Area Network) which is a system used widely for the control system network of a vehicle, there exists a mechanism for assigning a priority to data (a message).
For example, there is a mechanism for cancelling transmission of a message having a low priority when the transmission timings of a plurality of messages coincide with one another and these messages collide with one another.
However, when the transmission of the message having a low priority is started previously, a transmitting process of transmitting a message having a high priority is kept waiting until the transmission of the message having a low priority is completed.
In the control system network of a vehicle, there are many messages which are transmitted at predetermined periods. Therefore, a control system for avoiding collision of messages by using this feature is disclosed by the following patent reference 1.
In this control system, a device which serves as a master (referred to as a “master device” from here on) is prepared on the network, and the master device receives a list showing the transmission time periods of outgoing messages from a plurality of other devices (each referred to as a “slave device” from here on) and counts the transmission time period of each outgoing message, and, when the transmission time period of one of the outgoing messages coincides with that of another outgoing message, commands the corresponding slave devices to shift the phases of the transmission time periods.
In this case, each slave device needs to implement a function of changing the transmission timing of its outgoing message according to the command from the master device.